U.S. patent number 6,417,022 [Application Number 09/547,546] was granted by the patent office on 2002-07-09 for method for making long focal length micro-lens for color filters.
This patent grant is currently assigned to Taiwan Semiconductor Manufacturing Co., Ltd.. Invention is credited to Bi-Cheng Chang, Yu-Kung Hsiao, Kuo-Liang Lu, Sheng-Liang Pan.
United States Patent |
6,417,022 |
Hsiao , et al. |
July 9, 2002 |
Method for making long focal length micro-lens for color
filters
Abstract
A method for making long focal length micro-lens for color
filters in CMOS image sensor applications and device made by the
method are described. In the method, a layer of micro-lens material
is first spin coated on top of a color filter, patterned by a
photolithographic method into at least four discrete regions, and
preferably at least nine discrete regions for each micro-lens with
a pre-set spacing therein between. The discrete regions allow a
smaller volume of micro-lens material to be used for forming the
micro-lens in a subsequent reflow process. The micro-lens formed by
the present invention novel method has a focal length of at least 7
.mu.m, and preferably at least 10 .mu.m such that a 0.35 .mu.m
technology CMOS image sensor utilizing two or three layers of metal
conductors can be formed by the present invention method.
Inventors: |
Hsiao; Yu-Kung (Tao-Yuan,
TW), Pan; Sheng-Liang (Hsin Chu, TW),
Chang; Bi-Cheng (Hsin Chu, TW), Lu; Kuo-Liang
(Hsin-Chu, TW) |
Assignee: |
Taiwan Semiconductor Manufacturing
Co., Ltd. (Hsin Chu, TW)
|
Family
ID: |
24185076 |
Appl.
No.: |
09/547,546 |
Filed: |
April 12, 2000 |
Current U.S.
Class: |
438/70;
438/72 |
Current CPC
Class: |
H01L
27/14627 (20130101); H01L 27/14685 (20130101); H01L
27/14621 (20130101) |
Current International
Class: |
H01L
21/00 (20060101); H01L 021/00 () |
Field of
Search: |
;438/69,70,71,72,780 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chaudhari; Chandra
Attorney, Agent or Firm: Tung & Associates
Claims
What is claimed is:
1. A method for making long focal length micro-lens for color
filters comprising the steps of:
providing a semiconductor substrate having at least one photodiode
formed in a top surface;
forming at least two metal conductor layers with metal conductors
embedded in at least two insulating layers;
forming a photoresist layer on top of said at least two metal
conductor layers, said photoresist layer further comprises a color
agent therein for forming color filters of at least one of red,
green or blue color;
depositing a layer of micro-lens material on top of said
photoresist layer, patterning said layer of micro-lens material
into at least four discrete regions for each micro-lens with
pre-set spacing therein between; and
reflowing said at least four discrete regions for each micro-lens
and forming said micro-lens.
2. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of forming
said at least two metal conductor layers to a thickness of at least
7 .mu.m.
3. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of forming
said at least two metal conductor layers to a thickness of
preferably at least 10 .mu.m.
4. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of forming
said photoresist layer containing a color agent of a pigment or dye
therein for forming color filters of red, green and blue.
5. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
depositing a layer of micro-lens material of polymeric nature on
top of said photoresist layer.
6. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
depositing a layer of micro-lens material selected from the group
consisting of acrylic and epoxy novolac resin.
7. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
depositing a layer of micro-lens material that is transparent on
top of said photoresist layer.
8. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
patterning said layer of micro-lens material into at least four
discrete regions in square shape for each micro-lens.
9. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
patterning said layer of micro-lens material into at least four
discrete regions in elongated strips for each micro-lens.
10. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
patterning said layer of micro-lens material into at least nine
discrete regions in square shape for each micro-lens.
11. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of forming
said pre-set spacing between said at least four discrete regions to
larger than 0.01 .mu.m.
12. A method for making long focal length micro-lens for color
filters according to claim 1 further comprising the step of
providing said semiconductor substrate having at least one
photodiode formed in a top surface to a depth of at least 0.1
.mu.m.
Description
FIELD OF THE INVENTION
The present invention generally relates to a method for fabricating
micro-lens for color filters and devices fabricated and more
particularly, relates to a method for fabricating long focal length
micro-lens for color filters and devices fabricated by such
method.
BACKGROUND OF THE INVENTION
Color fillers have been used in image sensors for CCD or CMOS
applications for producing color images. An essential part of the
color filters is the multiplicity of micro-lenses that is formed on
top of the color filters in order to focus a light beam.
Micro-lenses can be fabricated by a variety of different methods.
one of the more frequently used methods is by a standard
photolithographic technique followed by reflowing photoresist
strips that are formed in cylinders or squares into hemispheres.
The technique is carried out by exposing the photoresist to process
temperatures in excess of the glass transition temperature of the
micro-lens material. At a temperature higher than the glass
transition temperature, the cohesive force in the material that
forms the solid patterns causes the surface area to be minimized
forming a hemisphere.
A typical process for forming a multiplicity of micro-lenses for
color filters is shown in FIGS. 1A and 1B. Referring now to FIG.
1A, wherein a process flow chart 10 for fabricating a micro-lens is
shown. The process flow-chart 10 corresponds to the structure 20
shown in FIG. 1B for a CMOS image sensor including a multiplicity
of micro-lenses 12 and a multiplicity of color filters 22. As shown
in FIG. 1B, a photodiode 24 is first formed in the surface of a
substrate 26, which also includes a series of metal conductors 28
covered by a layer of passivation 30 that forms an irregular upper
surface 32. The irregular upper surface 32 is then planarized by
depositing a layer of dielectric material 34 over the passivation
layer 30. As shown in FIG. 1A, after the planarization step 14 has
been performed, a subsequent step 16 is carried out to form a color
filter layer 16 providing red, green and blue color elements. As a
final step, a micro-lens spacer 36 is applied by step 18 following
which a micro-lens 12 is formed by step 38.
A more detailed view of the conventional color filter 20 is shown
in FIGS. 2A, 2B, 3A and 3B. FIG. 2A illustrates an enlarged,
cross-sectional view of the image sensor 20 before the micro-lens
reflow process, while FIG. 2B illustrates an enlarged,
cross-sectional view of the image sensor 20 after the micro-lens
reflow process. By using the conventional technology, the
micro-lens material layer (not shown) is first spin coated on the
surface of the spacer layer 36. The thinnest possible thickness for
the micro-lens material layer formed by using the presently
available spin coating technology is between about 1.3 .mu.m and
about 1.4 .mu.m. This is achieved at a spinning speed of about 4500
RPM. By using the conventional technology, the focal length of the
micro-lens fabricated is about 5.about.6 .mu.m for a 6 .mu.m
diameter micro-lens. The focal length achieved is not sufficient
for the new generation color filters, i.e. for 0.35 .mu.m
technology CMOS image sensors which require a focal length of at
least 7 .mu.m and preferably 10 .mu.m. The longer focal length is
necessary in order for a light beam to focus on the photodiode 24
that is formed in the substrate 26. A multiple number of
micro-lenses is shown in a plane view of FIG. 3B while an enlarged,
cross-sectional view of a single micro-lens formed by the
conventional method with a large diameter is shown in FIG. 3B.
The conventional CMOS image sensor shown in FIG. 2B has only one
metal layer 28 formed in a passivation layer 30 and a planarization
layer 34. In modern semiconductor devices used as image sensors, a
multiple number of metal layers, for instance 3 metal conductor
layers are utilized in a 0.35 .mu.m technology CMOS image sensor.
The small diameter of the micro-lens 12, which is contributed by
the large thickness of the micro-lens material spin-coated (FIG.
2A) cannot produce a micro-lens that has the necessary focal length
of at least 7 .mu.m and preferably at least 10 .mu.m. In order to
reduce the thickness of the micro-lens material, as shown in FIG.
2A, a significantly higher rotational speed must be utilized in a
spin coating process, i.e. a speed higher than 4500 RPM for
producing a thickness of 1.4 .mu.m micro-lens material. At such
high rotational speed, various other processing problems can be
caused which include a possible loss of wafer from the wafer
platform due to a break in vacuum used in holding the wafer. It is
therefore impossible to vary the processing parameters of a spin
coating process in order to provide a thinner layer of the
micro-lens material, or to produce a multiplicity of micro-lenses
each having a focal length longer than 7 .mu.m.
It is therefore an object of the present invention to produce a
long focal length micro-lens in an image sensor application that
does not have the drawbacks or shortcomings of conventional
methods.
It is another object of the present invention to provide a method
for making a long focal length micro-lens for use in color filters
that have a focal length of at least 7 .mu.m.
It is a further object of the present invention to provide a method
for making a long focal length micro-lens for colored filters that
is suitable for fabricating 0.35 .mu.m technology CMOS image
sensors.
It is another further object of the present invention to provide a
method for making a long focal length micro-lens for colored
filters that can be used for CMOS image sensors that have at least
two metal conductor layers.
It is still another object of the present invention to provide a
method for making a long focal length micro-lens for color filters
that is suitable for producing a micro-lens material layer of less
than 1 .mu.m thick.
It is yet another object of the present invention to provide a
method for making a long focal length micro-lens for color filters
suitable for use in CMOS image sensors wherein a micro-lens
material is first coated to a thickness of less than 1 .mu.m, and
then patterned and reflowed into micro-lenses having a focal length
larger than 7 .mu.m for 6 .mu.m diameter micro-lenses.
It is still another further object of the present invention to
provide a color filter that has a multiplicity of long focal length
micro-lenses built thereon wherein each of the multiplicity of
micro-lenses has a focal length of at least 10 .mu.m for 6 .mu.m
diameter micro-lenses.
It is yet another further object of the present invention to
provide a color filter that has a multiplicity of long focal length
micro lenses built thereon for a CMOS image sensor application that
contains at least two metal conductor layers with metal conductors
embedded in at least two insulating material layers.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for making long
focal length micro lenses for color filters and a device made by
such method are provided.
In a preferred embodiment, a method for making long focal length
micro-lenses for color filters can be carried out by the operating
steps of first providing a semiconductor substrate that has at
least one photodiode formed in a top surface; forming at least two
metal conductor layers with metal conductors embedded in at least
two insulating layers; forming a photoresist layer on top of the at
least two metal conductor layers, the photoresist layer further
includes a color agent therein for forming color filters of at
least one of red, green and blue colors; depositing a layer of
micro-lens material on top of the photoresist layer; patterning the
layer of micro-lens material into at least four discrete regions
for each micro-lens with pre-set spacing therein between; and
reflowing the at least four discrete regions for the micro-lens and
forming the micro-lens.
The method for making a long focal length micro-lens for color
filters may further include the step of forming the at least two
metal conductor layers to a thickness of at least 7 .mu.m, and
preferably to a thickness of at least 10 .mu.m. The method may
further include the step of forming the photoresist layer
containing a color agent of a pigment or dye therein for forming
color filters of red, green or blue. The method may further include
the step of depositing a layer of micro-lens material of polymeric
nature on top of the photoresist layer, or the step of depositing a
layer of micro-lens material selected from the group consisting of
acrylic and epoxy novolac resin, or the step of depositing a layer
of micro-lens material that is transparent on top of the
photoresist layer.
The method for making long focal length micro-lens for color
filters may further include the step of patterning the layer of
micro-lens material into at least four discrete regions in square
shape for each micro-lens. The method may further include the step
of patterning the layer of micro-lens material into at least four
discrete regions in elongated strips for each micro-lens, or the
step of patterning the layer of micro-lens material into at least
nine discrete regions in square shape for each micro-lens, or the
step of forming the pre-set spacing between the at least four
discrete regions to larger than 0.01 .mu.m. The method may further
include the step of providing the semiconductor substrate that has
at least one photodiode formed in a top surface to a depth of at
least 0.1 .mu.m.
The present invention is further directed to a color filter that
has a multiplicity of long focal length micro-lenses build thereon
including a semiconductor substrate that has at least one
photodiode formed in a top surface; at least two metal conductor
layers with metal conductors embedded in at least two insulating
layers; a photoresist layer on top of the at least two metal
conductor layers, the photoresist layer further includes a color
agent therein for forming color filters of at least one of red,
green or blue color; a multiplicity of micro-lenses on top of the
photoresist layer each positioned corresponding to one of the color
filters of at least one of red, green or blue color, the
multiplicity of micro-lenses each has a focal length of not less
than 7 .mu.m.
In the color filter that has a multiplicity of long focal length
micro-lenses built thereon, the multiplicity of micro-lenses each
having a focal length of preferably not less than 10 .mu.m. The at
least two metal conductor layers may include three metal conductor
layers with metal conductors embedded in three insulating material
layers, the at least one photodiode formed in the top surface of
the semiconductor substrate may have a thickness of at least 0.1
.mu.m. The at least two insulating material layers are formed of
dielectric materials. The color agent may be a dye or a pigment.
The color filters may be formed of red, green or blue color. The
layer of micro-lens material is formed of a polymeric material, a
transparent material, or an acrylic or epoxy novolac resin.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the present
invention will become apparent from the following detailed
description and the appended drawings in which:
FIG. 1A is a process flow chart for a conventional process for
forming a micro-lens on a color filter.
FIG. 1B is an enlarged, cross-sectional view of a conventional CMOS
image sensor with color filters and a multiplicity of micro-lenses
formed thereon.
FIG. 2A is an enlarged, cross-sectional view of a conventional
color filter with a micro-lens material deposited and patterned
thereon prior to the reflow process.
FIG. 2B is an enlarged, cross-sectional view of a conventional CMOS
image sensor with a multiplicity of micro-lenses built thereon.
FIG. 3A is an enlarged, plane view of a layer of conventional
micro-lens material patterned into squares.
FIG. 3B is an enlarged, cross-sectional view of a conventional
micro-lense formed after a reflow process.
FIG. 4A is an enlarged, plane view of the present invention
micro-lens material layer patterned on top of a color filter.
FIG. 4B is an enlarged, cross-sectional view of a present invention
micro-lens after a reflow process.
FIG. 4C is an enlarged, cross-sectional view of the present
invention micro-lens material layer after patterning by a
photolithographic process.
FIG. 5A is an enlarged, cross-sectional view of a present invention
color filter with a multiplicity of micro-lenses patterned on top
prior to a reflow process.
FIG. 5B is an enlarged, cross-sectional view of a present invention
CMOS image sensor with a multiplicity of present invention
micro-lenses formed thereon.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention discloses a method for making long focal
length micro-lenses for color filters for use in CMOS image sensors
and devices made by such method.
In the method for fabricating long focal length micro-lens for
color filters, after a layer of micro-lens material is first spin
coated on top of a photoresist layer, the layer of micro-lens
material is patterned by a photolithographic method into at least
four discrete regions, and preferably into at least nine discrete
regions for each micro-lens with a pre-set spacing therein between.
The pre-set spacing may be at least 0.01 .mu.m for a 6 .mu.m
diameter micro-lens. The discrete regions formed in the micro-lens
material layer may be an array of squares or a grating pattern of
strips. The discrete regions with spacing therein between
effectively reduce the amount of the micro-lens material available
for forming the micro-lenses during the reflow process. The larger
the spacing formed in-between the discrete regions, the lesser the
total material is available for forming the micro-lenses, and
therefore the larger the focal length of the micro-lenses formed.
The present invention novel method effectively reduce the volume of
micro-lens material available for forming the micro-lens and
therefore resulting in long focal length micro-lens. For a 6 .mu.m
diameter micro-lens, the focal length of the micro-lens obtained by
the present invention novel method can be at least larger than 7
.mu.m, or preferably at least larger than 10 .mu.m.
The present invention novel method eliminates the problem that
would otherwise be caused when attempting to form a thinner
micro-lens material layer on top of color filters by increasing the
spinning speed during a spin coating process. For instance, in
order to form a micro-lens material layer having a thickness less
than 1 .mu.m, very high spinning speed such as 7,000.about.8,000
RPM must be used. At such high RPM, a vacuum break may occur such
that the wafer is no longer held by vacuum on the wafer platform.
This results in severe wafer damage or a complete breakage of the
wafer.
The present invention is further directed to a color filter that
has a multiplicity of long focal length micro-lenses built thereon.
The multiplicity of micro-lenses formed on top of a photoresist
layer is each positioned corresponding to one of the color filters
of red, green or blue. The multiplicity of micro-lenses each has a
focus length of not less than 7 .mu.m, and preferably not less than
10 .mu.m. Such focus length of the micro-lenses is suitable for use
in 0.35 .mu.m technology CMOS image sensors wherein at least two or
three metal conductor layers are utilized which are embedded in at
least two or three insulating material layers having a total
thickness of about 7 .mu.m.
Referring now to FIG. 4A wherein an enlarged, plane view of a
present invention micro-lens material layer after patterning by a
photolithographic method is shown. The patterned micro-lens
material is shown in three separate embryos 40, 42 and 44 for
forming three separate micro-lenses of long focal length. It is
noted that each of the micro-lens embryo is divided into nine
discrete regions 46 of different sizes. The spacing 48 between the
regions 46 is pre-set at at least 0.01 .mu.m when a 6 .mu.m
diameter micro-lens is to be formed. For micro-lenses of larger
focal length, larger pre-set spacings 48 between regions 46 should
be used. After the embryos 40, 42 and 44 are patterned by a
standard photolithographic method, the embryos are reflown in a
reflow process at a temperature of about 160.degree. C. forming a
micro-lens such as that shown in FIG. 4B in an enlarged,
cross-sectional view. An enlarged cross-sectional view of the
region 46 in a micro-lens embryo that was divided into 25 regions
is further shown in FIG. 4C. It should be noted that the discrete
regions are reflown and joined together after a reflow process
conducted at about 160.degree. C. for a pre-determined length of
time.
Another enlarged, cross-sectional view of a present invention
color-filter 60 that has a multiplicity of micro-lenses 62 built
thereof is shown in FIG. 5A. It should be noted that the color
filter layer 22 may consist of a red filter 72, a green filter 74
and a blue filter 76. While the color filter layer 22 is embedded
in a spacer layer 36.
As shown in FIG. 5B of an enlarged, cross-sectional view of the
present invention CMOS image sensor 80, the focal length x that is
formed by the present novel invention structure is at least 7
.mu.m, and preferably at least 10 .mu.m to accommodate three
separate layers of metal conductors 28, 78 and 88. This structure
is frequently seen in a 0.35 .mu.m technology CMOS image sensor
application. The long focal length x of the micro-lens 62 therefore
allows an accurate focus on the photodiode 24 through the color
filter layer 22.
The present invention novel method divides a micro-lens material
layer into discrete areas with pre-set spacing therein between for
reducing the total volume of the micro-lens material and for
allowing a larger diameter micro-lens to be formed. The large
diameter micro-lens produces a larger focal length and enables the
present invention novel application that includes two or three
layers of metal conductors in a CMOS image sensor.
The present invention novel method and device made by the method
have therefore been amply described in the above description and in
the appended drawings of FIGS. 4A-5B.
While the present invention has been described in an illustrative
manner, it should be understood that the terminology used is
intended to be in a nature of words of description rather than of
limitation.
Furthermore, while the present invention has been described in
terms of a preferred embodiment, it is to be appreciated that those
skilled in the art will readily apply these teachings to other
possible variations of the inventions.
The embodiment of the invention in which an exclusive property or
privilege is claimed are defined as follows.
* * * * *